This invention relates generally to signal processing. More particularly, this invention relates to a dynamic signal pre-distortion technique utilizing Lagrange interpolation to improve data output from a nonlinear device.
Signal distortion produced by nonlinear devices, such as high-powered amplifiers, is an ongoing problem. Nonlinear devices commonly distort a signal and cause spectral side lobes. It is known to compensate for a nonlinear device by pre-distorting the signal processed by the nonlinear device. The pre-distortion is generally in the form of an inverse function of the nonlinear device. Since operating device temperature and aging may cause the nonlinear function to change with time, the pre-distortion should adapt in a dynamic manner to account for these varying conditions.
Many existing pre-distortion schemes operate through the use of look-up tables. That is, the inverse function to provide the requisite pre-distortion is produced from a look-up table that has pre-distortion output signals for a set of input signals. The problem with this approach is that if the amplitude deviation of the input signal is large, the memory required to implement the look-up table is large. This results in additional system expense and also precludes efficient dynamic updates of look-up table values.
In view of the foregoing, it would be highly desirable to provide an improved technique for compensating for nonlinear devices. More particularly, it would be highly desirable to provide a dynamic pre-distortion technique that does not rely upon look-up tables and otherwise operates to quickly provide dynamic pre-distortion for a nonlinear device.
The invention includes an apparatus for pre-distorting a signal. The apparatus includes input nodes to receive an in-phase input signal and a quadrature input signal. Feedback nodes receive an in-phase feedback signal and a quadrature feedback signal. Output nodes transmit an in-phase pre-distorted signal and a quadrature pre-distorted signal. A Lagrange interpolation digital pre-distortion circuit is connected to the input nodes, the feedback nodes, and the output nodes. The Lagrange interpolation digital pre-distortion circuit is configured to perform a Lagrange interpolation on the in-phase input signal, the quadrature input signal, the in-phase feedback signal, and the quadrature feedback signal to produce the in-phase pre-distorted signal and the quadrature pre-distorted signal.
The invention also includes a method of pre-distorting a signal. The method includes receiving an in-phase input signal, a quadrature input signal, an in-phase feedback signal, and a quadrature feedback signal. A Lagrange interpolation is performed on the in-phase input signal, the quadrature input signal, the in-phase feedback signal, and the quadrature feedback signal to produce an in-phase pre-distorted signal and a quadrature pre-distorted signal.
The invention provides dynamic pre-distortion without the expense and performance shortcomings associated with look-up tables. The Lagrange interpolation polynomials utilized in accordance with the invention approximate an inverse distortion function with a small number of data points. This approach allows for a dynamic update of sampling points and the interpolation of data between data points.